Modern oil field operators demand access to a great quantity of information regarding the parameters and conditions encountered downhole. Such information typically includes characteristics of the earth formations traversed by the borehole and data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging,” can be performed by several methods including wireline logging, tubing-conveyed logging, and “logging while drilling” (LWD).
In wireline logging, a sonde is lowered into the borehole after some or all of the well has been drilled. The sonde hangs at the end of a long cable or “wireline” that provides mechanical support to the sonde and also provides an electrical connection between the sonde and electrical equipment located at the surface of the well. In accordance with existing logging techniques, various parameters of the earth's formations are measured and correlated with the position of the sonde in the borehole as the sonde is pulled uphole.
Tubing-conveyed logging is similar to wireline logging, but the sonde is mounted on the end of a tubing string. The rigid nature of the tubing string enables the tubing-conveyed sonde to travel where it would be difficult to send a wireline sonde, e.g., along horizontal or upwardly-inclined sections of the borehole. The tubing string can include embedded conductors in the tubing wall for transporting power and telemetry, or a wireline cable can be fed through the interior of the tubing string, or the sonde can simply store data in memory for later retrieval when the sonde returns to the surface.
In LWD, the drilling assembly includes sensing instruments that measure various parameters as the formation is being drilled, thereby enabling measurements of the formation while it is less affected by fluid invasion. While LWD measurements are desirable, drilling operations create an environment that is generally hostile to electronic instrumentation, telemetry, and sensor operations.
One of the instruments that has been employed in each of these environments is a nuclear magnetic resonance (NMR) logging tool. NMR tools operate by using an imposed static magnetic field, B0, to give nuclei with non-zero nuclear spin (non-zero angular momentum) split energy levels. Since lower energy levels are preferred, an ensemble of nuclei will exhibit an anisotropic distribution of energy states, giving the nuclear spins a preferential polarization parallel to the imposed field. This state creates a net magnetic moment, producing a bulk magnetization. The nuclei (primarily hydrogen nuclei) converge upon their equilibrium alignment with a characteristic exponential relaxation time constant. When this convergence occurs after the nuclei have been placed in a cooperative initial state (discussed below), it is known as recovery. The time constant for recovery is called the “spin-lattice” or “longitudinal” relaxation time T1.
During or after the polarization period, the tool applies a perturbing field, usually in the form of a radio frequency electromagnetic pulse whose magnetic component, B1, is perpendicular to the static field B0. This perturbing field moves the orientation of the magnetization into the transverse(perpendicular) plane. The frequency of the pulse can be chosen to target specific nuclei (e.g., hydrogen). The polarized nuclei are perturbed simultaneously and, when the perturbation ends, they precess around the static magnetic field gradually re-polarizing to align with the static field once again while losing coherence in the transverse plane (T2 relaxation). The precessing nuclei generate a detectable radio frequency signal that can be used to measure statistical distributions of T1, T2, porosities, and/or diffusion constants.
The transverse relaxation time (also called the “spin-spin” relaxation time) represents how quickly the transverse plane magnetization disperses through de-phasing and magnitude loss. Forces aligned with the transverse plane contribute to non-adiabatic, non-reversible, relaxation while those aligned with the static field contribute to adiabatic, reversible relaxation. The intrinsic transverse time relaxation constant, i.e., relaxation that is solely attributable to non-adiabatic effects, is labeled as “T2” and it is solely a property of the substance. The measured transverse relaxation time constant, however, is also influenced by environmental factors and field inhomogeneities that cause the magnetization to dephase. The time constant for all transverse relaxation processes together (intrinsic and environmental) is labeled as “T2*”.
To isolate the intrinsic T2 subsequent RF pulses can be applied to invert the spin phases and cause the net magnetization to gradually refocus into phase, thus rebuilding the induced signal to create “spin echoes”. After each echo signal peaks (at the time when the nuclei are back in phase), the signal begins to decay again in the same manner as before. Another follow-up pulse can be used to again reverse the instantaneous phases and thereby rebuild the signal again to a subsequent echo.
By using a series of follow-up pulses, the signal is periodically rebuilt after each dephasing, although each rebuilding is to a slightly lesser peak amplitude due to the intrinsic losses in magnetization so eventually the echo signals die out completely. The time constant associated with the decay of the recurring spin echo amplitudes approaches the transverse relaxation time T2. (Molecular motion through gradients cause another irreversible spreading of the magnetization so the true T2 can only be measured in a perfectly uniform magnetic field.)
A sequence of refocusing pulses is known in the art as the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence. It is most frequently used for measuring T2 distributions. A popular method for measuring T1 distribution employs observing the effect of different recovery time spacings between separate CPMG experiments. Other methods utilizing consecutively spaced RF perturbations followed by a CPMG sequence can also be employed to probe the magnetization build up. As is well known in the industry, the relaxation time distribution information from either T2 or T1 can be readily converted into measurements of porosity (i.e., the relative amount of void space in the formation), hydrocarbon saturation (i.e., the relative percentage of hydrocarbons and water in the formation fluid), and permeability (i.e., the ability of formation fluid to flow from the formation into the well bore). For a more comprehensive overview of the NMR technology including logging methods and various tool designs, the interested reader is directed, for example, to the book by Coates et al. entitled “NMR Logging: Principles and Applications” distributed by Gulf Publishing Company (2000), and hereby incorporated herein by reference for background. Additional description of NMR logging techniques is provided, for example, in U.S. Pat. Nos. 4,710,713; 4,717,876; 4,717,877; 4,717,878; 4,939,648; 5,055,787; 5,055,78; 5,212,447; 5,280,243; 5,309,098; 5,412,320; 5,517,115, 5,557,200; 5,696,448; 5,936,405; 6,005,389; 6,023,164; 6,051,973; 6,107,796; 6,111,408; 6,242,913; 6,255,819; 6,268,726; 6,362,619; 6,512,371; 6,525,534; 6,531,868; 6,541,969; 6,577,125; 6,583,62; 6,646,437; 6,717,404; and 7,463,027 which are hereby incorporated herein by reference.
The signals obtained by nuclear magnetic resonance (NMR) logging tools are weak, e.g., on the order of nanovolts. Signal-to-noise ratio (SNR) is a primary concern when judging the performance of such tools. SNR can be increased by combining multiple measurements and/or by increasing the measurement volume. The measurement volume is a function of magnetic field homogeneity and the height of the transmit and receive antennas. However, the vertical resolution of the tool is also a function of the height of the receive antenna and (due to the tool's motion) the number of measurements that are combined. Existing logging tools have to find a feasible tradeoff between vertical resolution and SNR and hence are not able, under normal operating conditions, to provide a high vertical resolution together with a high SNR.
While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the disclosure to the particular illustrated embodiments, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the scope of the appended claims.